Broadcast channels in the FM band (e.g., about 76 MHz to 108 MHz) are often transmitted with an FM stereo multiplex (MPX) format where the MPX signal includes left (L) channel and right (R) channel information that can then be used by an FM receiver to produce stereo audio outputs. In particular, the FM stereo MPX signal format includes L+R (left-plus-right) information, L−R (left-minus-right) information and a 19 KHz pilot tone. With respect to the center frequency of the broadcast channel, the L+R information lies in a band between 30 Hz and 15 KHz from the center frequency in the broadcast channel. The L−R information lies in two bands on either side of 38 KHz from the center frequency of the broadcast channel, namely a first band between 23 KHz and 38 KHz and a second band between 38 KHz and 53 KHz. And the pilot tone sits at 19 KHz from the center frequency of the broadcast channel. Most FM receivers will produce a full stereo output by generating a left (L) channel audio output by adding the L+R and L−R signals ((L+R)+k*(L−R)=2L when k=1) and a right (R) channel audio output signal by subtracting the L+R and L−R signals ((L+R)−k*(L−R)=2R when k=1). If a mono output is desired or selected, most FM receivers will drive k to 0 in order to pass the full (L+R) signal to both the left (L) channel and the right (R) channel. In addition, varying degrees of a blend from stereo to mono can also be provided by adjusting or varying k from 1 to 0 to provide the desired level of a blend from stereo to mono.
FM broadcast band receivers can suffer from noise due to strong nearby blockers and/or other sources or conditions, and this noise or interference will often show up as static in the stereo audio output for the tuned FM channel. Techniques have been proposed before to mitigate this stereo noise. One prior technique is to use a receive signal strength indicator (RSSI) to blend the audio output from stereo to mono when the RSSI indicates poor signal strength. Another prior technique is to use a signal-to-noise ratio (SNR) measurement of the incoming signal to blend from stereo to mono when the SNR is low. The SNR measurement can be made, for example, by analyzing the amplitude modulation in the received FM broadcast signal. In addition, other solutions have proposed blending to mono based upon an analysis of variations in the FM pilot tone and/or an analysis of high frequency components that are above the frequencies for the FM information in the tuned signals.
FIG. 1 illustrates a plot of audio noise floor versus received RF strength that is typical of the results of a conventional prior art technique for blending from stereo to mono based on SNR measurement of a received incoming FM radio frequency (RF) signal. As shown in FIG. 1, the audio output is selected to transition between full stereo (k=1) and full mono (k=0) based on received RF level. In particular, audio output is selected to be full stereo (k=1) for received RF level values greater than or equal to minimum full stereo RF level LS and is selected to be full mono (k=0) for received RF level values less than or equal to maximum full mono RF level LM. At received RF levels between LS and LM the audio output is blended between stereo and mono as shown. In FIG. 1, the minimum full stereo level (LS) intersects the full stereo (k=1) curve at an audio noise threshold (NT) where blending from full stereo to mono begins. Threshold NT represents the noise level above which full stereo audio output is undesirable and blending between stereo and mono should occur to make the listening experience more pleasurable. This is because for a given noise level, more noise is present in the L−R data than in the corresponding L+R data.
Still referring to FIG. 1, the typical prior art transition from full stereo to full mono between LS and LM occurs in a manner that allows the audio output curve to exceed the selected audio noise threshold (NT) while in full or partial stereo in the blend region immediately below LS before eventually dropping beneath NT in the portion of the blend region closer to LM. This produces a “noise hump” in the composite noise curve, i.e., the non-coherent sum of the noise from the L+R channels and the L−R channels such that the noise when in full stereo is approximately the L+R noise. This noise hump is produced in the composite noise curve above NT in the blend region between LS and LM as shown. The consequence of this hump is increased noise level in parts of the blend region closer to LS while audio output is in full or partial stereo mode. To compensate for this hump in the composite noise curve where audio output noise would otherwise exceed NT, blending from full stereo must be initiated at a higher RF level (LS) than illustrated in FIG. 1 to ensure that the audio output remains below the desired NT value while in the blend region. Thus, in the example of FIG. 1, if the actual desired audio noise threshold NT corresponding to LS is selected as −40 dB, the composite noise curve will exceed −40 dB in the shaded area of the hump near LS while in full or partial stereo mode unless a substitute value of LS is selected that corresponds to an audio noise floor that is sufficiently below the actual desired value of NT so that the audio output does not exceed the actual desired NT while in full or partial stereo mode.